Permanent magnets based on compositions containing iron, neodymium and/or praseodymium, and boron are now known and in commercial usage. Such permanent magnets contain as an essential magnetic phase grains of tetragonal crystals in which the proportions of iron, neodymium and boron (for example) are exemplified by the empirical formula Nd.sub.2 Fe.sub.14 B. These magnet compositions and methods for making them are described by Croat in U.S. Pat. No. 4,802,931 issued Feb. 7, 1989. The grains of the magnetic phase are surrounded by a second phase that is typically rare earth-rich, as an example neodymium-rich, as compared with the essential magnetic phase. It is known that magnets based on such compositions may be prepared by rapidly solidifying (such as by melt spinning) a melt of the composition to produce fine grained, magnetically isotropic platelets of ribbon-like fragments. Magnets may be formed from these isotropic particles by practices which are known and which will be discussed further herein. The isotropic particles have many useful applications, but as recognized by the art there is also a need for an anisotropic powder with a high coercivity at room temperature.
Lee, U.S. Pat. No. 4,782,367, issued Dec. 20, 1988, demonstrated that the melt-spun isotropic powder can be suitably hot pressed and/or hot worked and plastically deformed to form high strength anisotropic permanent magnets. Such magnets have excellent magnetic properties. Maines et al, U.S. Pat. No. 4,842,656, issued Jun. 27, 1989, demonstrated that an anisotropic powder having a magnetic coercivity of at least about 1,000 Oersteds could be formed from the magnetic bodies of Lee by pulverizing the hot-worked magnetic bodies to a powder. The resultant particles of the powder are both magnetically anisotropic and have retained appreciable magnetic coercivity.
Further work in this area has focused on improving the coercivity of the anisotropic powder so as to be capable of forming magnetic bodies having improved magnetic properties. U.S. Pat. No. 4,952,239 to Tokunaga et al, issued Aug. 28, 1990, improves the coercivity of the anisotropic particles by appropriately heat treating the hot-worked magnetic body prior to forming the particles by pulverization. U.S. Pat. No. 4,983,232 to Endoh et al, issued Jan. 8, 1991, attempts to improve the coercivity of the anisotropic particles by appropriate additions of gallium to the magnetic composition, as well as by heat treating the hot-worked magnetic body prior to pulverization for formation of the particles.
Although these prior art methods have worked satisfactorily to produce anisotropic particles having coercivities of at least about 1,000 Oersteds at room temperature, it would be desirable to even further enhance the coercivity of these anisotropic particles.